In Mahabharat, Karna was the son of Lord Sun. The Sun has given him Kavach (Breastplate Armor) & Kundal (Ear-rings) to protect him. This Kavach & Kundal were attached to his body. As long as this Kavach & Kundal were protecting Karna, no weapon could harm him.
Karna has a habit of giving donation daily after the giving ardhya to Sun in the morning. When the Mahabharata's fight was about to begin, Indra went to Karna and asked for his Kavach & Kundal. Karna gave his Kavach & Kundal to Indra.
But then, What Indra did with Kavach & Kundal?